[backport] chainHead/fix: Report bestBlock events only for newBlock reports (#5527)

[lexnv]

This backports original PR: https://github.com/paritytech/polkadot-sdk/pull/5527 to the release branch

The https://github.com/paritytech/polkadot-sdk/issues/5512 has surfaced that we reported a `BestBlock` event for a block not previously reported via `NewBlock`.

This is because of a race between:
- the stream of events that announces new blocks
- `self.client.info().best_block`

It is possible that `client.info()` contains newer information than the information polled from the block stream (that may be lagging).

To mitigate this, instead of relying on the client's info use the last finalized block to emit a new event.

There are two cases when a new best block event is emitted:
- The best block is in the pruned list and is reported immediately
- The best block is not a descendant of the last finalized block 

Closes: https://github.com/paritytech/polkadot-sdk/issues/5512 

Thanks @jsdw and @josepot for helping debug this 🙏 

cc @paritytech/subxt-team

[github-actions[bot]]

This pull request is amending an existing release. Please proceed with extreme caution, as to not impact downstream teams that rely on the stability of it. Some things to consider:

• Backports are only for 'patch' or 'minor' changes. No 'major' or other breaking change.
• Should be a legit fix for some bug, not adding tons of new features.

• Must either be already audited or not need an audit.

  Emergency Bypass

  If you really need to bypass this check: add validate: false to each crate in the Prdoc where a breaking change is introduced. This will release a new major version of that crate and all its reverse dependencies and basically break the release.